Position sensing is used to electronically monitor the position or movement of a mechanical component. The position sensor produces an electrical signal that varies as the position of the component in question varies. Electrical position sensors are included in many products. For example, position sensors allow the status of various automotive components to be monitored and controlled electronically.
A position sensor needs to be accurate, in that it must give an appropriate electrical signal based upon the position measured. If inaccurate, a position sensor may hinder the proper evaluation and control of the position of the component being monitored.
Typically, it is also a requirement that a position sensor be adequately precise in its measurement. However, the precision needed in measuring a position will obviously vary depending upon the particular circumstances of use. For some purposes, only a rough indication of position is necessary; for instance, an indication of whether a valve is mostly open or mostly closed. In other applications, more precise indication of position may be needed.
A position sensor should also be sufficiently durable for the environment in which it is placed. For example, a position sensor used on an automotive valve may experience almost constant movement while the automobile is in operation. Such a position sensor should be constructed of mechanical and electrical components adequate to allow the sensor to remain sufficiently accurate and precise during its projected lifetime, despite considerable mechanical vibrations and thermal extremes and gradients.
In the past, position sensors were typically of the “contact” variety. A contacting position sensor requires physical contact to produce the electrical signal. Contacting position sensors typically consist of potentiometers that produce electrical signals which vary as a function of the component's position. Contacting position sensors are generally accurate and precise. Unfortunately, the wear due to contact during movement has limited their durability. Also, the friction resulting from the contact can degrade the operation of the component. Further, water intrusion into a potentiometric sensor can disable the sensor.
One advancement in sensor technology has been the development of non-contacting position sensors. A non-contacting position sensor (“NPS”) does not require physical contact between the signal generator and the sensing element. Instead, an NPS utilizes magnets to generate magnetic fields that vary as a function of position, and devices to detect varying magnetic fields to measure the position of the component to be monitored. Often, a Hall Effect device is used to produce an electrical signal that is dependent upon the magnitude and polarity of the magnetic flux incident upon the device. The Hall Effect device may be physically attached to the component to be monitored and thus moves relative to the stationary magnet(s) as the component moves. Conversely, the Hall Effect device may be stationary with the magnet(s) affixed to the component to be monitored. In either case, the position of the component to be monitored can be determined by the electrical signal produced by the Hall Effect device.
The use of an NPS presents several distinct advantages over the use of a contacting position sensor. Because an NPS does not require physical contact between the signal generator and the sensing element, there is less physical wear during operation, resulting in greater sensor durability. The use of an NPS is also advantageous because the lack of any physical contact between the items being monitored and the sensor itself results in reduced drag.
While the use of an NPS presents several advantages, there are also several disadvantages that must be overcome in order for an NPS to be a satisfactory position sensor for many applications. Magnetic irregularities or imperfections can compromise the precision and accuracy of an NPS. The accuracy and precision of an NPS can also be affected by the numerous mechanical vibrations and perturbations likely to be experienced by the sensor. Because there is no physical contact between the item to be monitored and the sensor, it is possible for them to be knocked out of alignment by such vibrations and perturbations. A misalignment can result in the measured magnetic field at any particular location not being what it would be in the original alignment. Because the measured magnetic field can be different than that when properly aligned, the perceived position can be inaccurate. Linearity of magnetic field strength and the resulting signal is also a concern.
Magnetic position sensors of the prior art also require special electronics to account for changes in the magnetic field with temperature. The field generated by a magnet changes with temperature and the sensor must be able to differentiate between changes in temperature and changes in position.